This invention relates to the field of power converter, in particular to the field of feedback compensation in complementary driven half-bridge converters.
Zero voltage switching reduces switching losses significantly and greatly improves converter efficiency especially when the bridge voltage is high and capacitance loss is significant. Furthermore, Zero voltage switching allows higher switching frequency for the bridge switches as well as use of smaller components.
Placing a DC blocking capacitor on the primary side of a converter results in an improved half bridge topology that provides zero voltage switching. This enables use of the complementary driving method, together with inductance on the primary side, for zero voltage switching of half-bridge switches.
Despite its many advantages, the complementary driving method has not been widely employed in half-bridge converters due to the complexity encountered in actual implementations. Small signal analysis of a circuit for a typical half-bridge converter reveals that the primary DC blocking capacitor and magnetizing inductance of the main transformer produce a second order resonant circuit. On the secondary side, output filter inductor and capacitor produce another resonant circuit. Together these two circuits form a fourth order system that is complex and consequently difficult to stabilize.
A number of researchers have addressed this problem but without forwarding a satisfactory solution. For instance, Korotkov et al. showed that the phase shift approaches 360 degrees when the both resonant circuits take effect with the aid of the theoretical Bode plot and the phase plot on the transfer function. Korotkov et al., xe2x80x9cSmall-signal modeling of soft-switched asymmetrical half-bridge DC/DC converterxe2x80x9d in the Applied Power Electronics Conference and Exposition on pages 707-11 in 1995. This phase shift can result in stability problems with use of negative feedback for regulating the output voltage.
While it is possible to solve the stability problem by rolling off the open loop gain to less than one before both of the two resonant circuits take effect this results in other undesirable consequences. For instance, this greatly impairs the dynamic response of the converter along with additional limitations in the design of the feedback compensation network.
Sebastian et al. suggested another solution in the paper titled xe2x80x9cSmall-signal modeling of the half-bridge complementary-control DC-to-DC converterxe2x80x9d in the Power Electronics Congress of 1995. Accordingly, ensuring a much greater resonant frequency for the output LC filter than the circuit comprising the primary magnetizing inductance and the DC blocking capacitor reduces the phase shift of asymmetrical half-bridge converter to less than 180 degrees. However, this method has limited applicability due to the limitation placed on the output filter design resulting in incompatibility with many normal converter specifications.
The present invention provides a system and method to reduce the aforementioned complexity enabling better regulation of the output voltage in a half-bridge converter with complementary drive. The system and method taught by the present invention do not require limitations on the choice of resonant frequencies in the half-bridge or a fall-bridge converter. In example embodiments of the invention, the voltage regulating feedback loop sees only the second order output inductor-capacitor filter. Therefore, ordinary second-order compensation can stabilize the loop and provide optimization more readily for better dynamic response.
An illustrative embodiment of the present invention includes modulation of a voltage ramp signal or an output error feedback voltage as a function of the voltage across the DC blocking capacitor. A comparator of the Pulse Width Modulation (PWM) controller controlling the power switches receives the modulated signal. This inner compensation loop eliminates the effect of voltage variation across the DC blocking capacitor.
Moreover, the present invention allows furher optimization of the dynamic response of half-bridge converters and full-bridge converters. This follows, in part, from many embodiments in accordance with the present invention employing simple second-order compensation and conventional techniques to stabilize the output voltage-regulating loop of half-bridge converters with complementary drive.
Thus, possible variations include, without limitation, a controlled current source providing a current as a function of the DC blocking capacitor voltage of a half-bridge converter to produce the voltage ramp signal for the PWM controller. Alternatively, the voltage of a winding coupled to the main transformer of the half-bridge converter modulates the output voltage error signal followed by feed the error signal into the PWM controller. The winding has voltage proportional to the voltage across the DC blocking capacitor and in a desired phase to provide inner loop compensation. Another embodiment illustrates the present invention in full-bridge converters.
These and other aspects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and from the accompanying drawings.